The theory predicts that preferred mate choices have evolved to focus on reproductive potential and reproductive investment of members of the opposite sex.
This theory predicts both intrasexual selection and intersexual choice due to differences in parental investment; typically there is competition among members of the lower investing sex (generally males) over the parental investment of the higher investing sex (generally females) who will be more selective in their mate choice.
A study conducted by David Buss investigated sex differences in mate preferences in 37 cultures with 10,047 participants.
In all 37 cultures it was found that males preferred females younger than themselves and females preferred males older than themselves.
Male chimpanzees tend to prefer older females than younger and it is suggested that specific cues of female mate value are very different to humans.
Buss attributed the young age preference for females to the cues that youth has.
There are also alternative social theories for age differences in relationships as well as suggested reasons for 'alternative' age-hypogamous relationships.
The study found very few instances of older women pursuing much younger men and vice versa.
Within sexual selection Darwin identified a further two mechanisms which are important factors in the evolution of sex differences (sexual dimorphism): intrasexual selection (involve competition with those of the same sex over access to mates) and intersexual choice (discriminative choice of mating partners).
Age-disparity relationships have been documented for most of recorded history and have been regarded with a wide range of attitudes dependant on sociocultural norms and legal systems. Relationships with age disparity of all kinds have been observed with both men and women as the older or younger partner.
In various cultures, older men and younger women often seek one another for sexual or marital relationships.